1. Field of the Invention
The present invention relates to an exhaust gas purifying method of internal combustion engine capable of suppressing degradation of exhaust purification efficiency of NOx-occluding reduction-type exhaust purifying catalysts due to SOx poisoning and thereby achieving higher purification efficiency of exhaust gas.
2. Related Background Art
The exhaust gas from an internal combustion engine is purified by an emission purifying catalyst such as a three way catalyst and then discharged to the atmosphere. The NOx-occluding reduction-type exhaust purifying catalysts have been also used as a kind of such exhaust purifying catalysts. The NOx-occluding reduction-type exhaust purifying catalysts are catalysts that occlude nitrogen oxides [NOx] under an excess condition of oxygen [O2] in exhaust gas but release the occluded NOx under a low-content condition of O2 in exhaust gas to effect reduction thereof (whereupon carbon monoxide [CO] and hydrocarbons [HC] in the exhaust gas are oxidized).
The exhaust purification efficiency can be increased more when such NOx-occluding reduction-type exhaust purifying catalysts are used so as to occlude NOx in exhaust gas during lean operation but release and reduce the occluded NOx during stoichiometric or rich operation. These NOx-occluding reduction-type exhaust purifying catalysts are useful in increasing the exhaust purification efficiency of lean-burn engines, which are designed positively to employ the lean operation as against ordinary engines, and thus they are also contributing to improvement in fuel consumption.
These NOx-occluding reduction-type exhaust purifying catalysts have the nature of occluding sulfur oxides [SOx] more stably than NOx. The SOx in exhaust gas are produced by oxidation of sulfur component included in fuel or engine oil, during combustion in the internal combustion engine. Although the quantity of sulfur component is very small in fuel or engine oil, the amount of sulfur component occluded becomes gradually increasing as accumulated in the catalyst, because it is stably occluded in the NOx-occluding reduction-type exhaust purifying catalysts. Once the NOx-occluding reduction-type exhaust purifying catalysts occlude a lot of SOx, they will disable the proper occlusion and release-reduction of NOx. This is the so-called xe2x80x9cSOx poisoningxe2x80x9d in the NOx-occluding reduction-type exhaust purifying catalysts.
In the conventional NOx-occluding reduction-type exhaust purifying catalysts, most of occlusion capacity in mint condition is applied to the occlusion of NOx, whereas after the SOx poisoning only small part of occlusion capacity is applied to the occlusion of NOx. If this SOx poisoning is suppressed, it will become feasible to increase the occlusion capability and release capability of NOx and greatly enhance the exhaust purifying performance of the NOx-occluding reduction-type exhaust purifying catalysts. There are known techniques for suppressing the SOx poisoning of the NOx-occluding reduction catalysts, e.g., one described in Japanese Patent Application Laid-Open No. 2000-27712, but its effect is not satisfactory yet. The inventors conducted research toward further improvement in the exhaust purification performance and accomplished the present invention.
It is, therefore, an object of the present invention to provide an exhaust purifying method of internal combustion engine capable of suppressing the SOx poisoning of NOx-occluding reduction-type exhaust purifying catalysts, thereby further increasing the purification efficiency of exhaust gas.
The present invention is a method of purifying exhaust gas from an internal combustion engine, wherein by use of a sulfur solidifier comprising a metal element having a function of oxidizing sulfur component, and a basic metal element, the sulfur component being the cause of SOx poisoning is solidified before the exhaust gas flows into an NOx-occluding reduction-type exhaust purifying catalyst placed on an exhaust path.
According to the present invention, the sulfur solidifier solidifies the sulfur component (incl. sulfur oxides) before the exhaust gas flows into the NOx-occluding reduction-type exhaust purifying catalyst placed on the exhaust path, which can suppress the SOx poisoning of the NOx-occluding reduction-type exhaust purifying catalyst and further increase the purification efficiency of exhaust gas. A significant point here is that the aforementioned sulfur solidifier comprises the metal element having the function of oxidizing the sulfur component (incl. sulfur oxides), and the basic metal element. This permits the solidifier to effectively solidify the sulfur component (incl. sulfur oxides) being the cause of SOx poisoning of the exhaust purifying catalyst, which ensures the improvement in the purification performance.
It is preferable herein to preliminarily mix the sulfur solidifier in fuel. This premixing of the sulfur solidifier in fuel facilitates control of the ratio of the sulfur solidifier to fuel, thus contributing to increase in solidification efficiency of the sulfur component.
In another preferred embodiment, the sulfur solidifier can be added in an intake path, in a combustion chamber, or in the exhaust path, separately from the fuel. In this configuration, since the sulfur solidifier is added in the intake path, in the combustion chamber, or in the exhaust path separately from the fuel, it is feasible to select the position and timing of addition suitable for the solidification reaction and to adjust a loading amount according to the operational status of the internal combustion engine or the like (also including a situation where no load of the solidifier is desired), thus contributing to increase in the solidification efficiency of the sulfur component.
The metal element having the function of oxidizing the sulfur component (incl. sulfur oxides) in the sulfur solidifier, is preferably a transition element.
In another aspect, the basic metal element in the sulfur solidifier is preferably an alkali metal element or an alkaline earth metal element. The basic metal element being the alkali metal element or the alkaline earth metal element is prone to react with the sulfur component and form salts (solid matter), and is thus able to increase the solidification efficiency of the sulfur component.
Further, the basic metal element in the sulfur solidifier is preferably an alkali metal element having an atomic number equal to or higher than the atomic number of potassium. When the basic metal element is an alkali metal element having an atomic number equal to or higher than the atomic number of potassium, it is particularly easy to react with the sulfur component to form salts (solid matter), and thus is capable of further increasing the solidification efficiency of the sulfur component.
Further, it is preferable to make the solidified sulfur component held on the exhaust purifying catalyst and act as a catalytic component. When the solidified sulfur component is made to be held on the exhaust purifying catalyst and to act as a catalytic component in this way, the occlusion capability of the exhaust purifying catalyst can be maintained high. As a result, it is feasible to further enhance the exhaust purifying performance and suppress deterioration of the exhaust purifying catalyst.
When the sulfur solidifier is added to the intake air, air/fuel mixture, or recirculated exhaust gas before combustion in the cylinder of the internal combustion engine, it becomes feasible to adequately adjust the loading amount of the sulfur solidifier and adjust the loading timing. For example, when the sulfur solidifier is included in fuel, the loading amount and time of the sulfur solidifier are limited by injected fuel quantity and fuel injection timing. When the solidifier is added to the intake air, air/fuel mixture, or recirculated exhaust gas, it is feasible to effectively promote the solidification of the sulfur component (incl. sulfur oxides) under high temperature and high pressure conditions during combustion or immediately after combustion in the cylinder, which is convenient to increase in the solidification efficiency. On this occasion, the sulfur solidifier is added to the intake air, the air/fuel mixture, or the recirculated exhaust gas, on the intake path or to the recirculated exhaust gas on an exhaust gas recirculation path routed from the exhaust path to the intake path.
The sulfur solidifier preferably contains the basic metal in an amount greater than a chemical equivalent to the sulfur component to be solidified. This allows the sulfur component to be solidified efficiently.
Another exhaust purifying method of internal combustion engine according to the present invention is a method of purifying exhaust gas from an internal combustion engine, wherein upstream of an NOx-occluding reduction-type exhaust purifying catalyst placed on an exhaust path, sulfur component (incl. sulfur oxides) being the cause of SOx poisoning is solidified by a sulfur solidifier comprising a basic metal element and wherein the sulfur component solidified is made to be held on the exhaust purifying catalyst and to act as a catalytic component.
According to the present invention, the sulfur solidifier solidifies the sulfur component (incl. sulfur oxides) being the cause of SOx poisoning before the exhaust gas flows into the NOx-occluding reduction-type exhaust purifying catalyst placed on the exhaust path, which can suppress the SOx poisoning of the NOx-occluding reduction-type exhaust purifying catalyst and further increase the purification efficiency of exhaust gas. A significant point herein is that the solidified sulfur component is made to be held on the exhaust purifying catalyst and to act as a catalytic component, whereby the occlusion capability of the exhaust purifying catalyst can be maintained high. This permits further improvement in the exhaust purifying performance and suppression of deterioration of the exhaust purifying catalyst.
Here the exhaust purifying catalyst preferably has a wall flow structure. This structure makes it easier to uniformly carry the solidified sulfur component inside the exhaust purifying catalyst and to react the exhaust gas with the sulfur component functioning as a catalyst on the exhaust purifying catalyst, which can improve the purifying performance of exhaust gas.
After held on the exhaust purifying catalyst, the solidified sulfur component thereon is preferably placed under high temperature and rich conditions to make the basic metal element carried on the exhaust purifying catalyst. According to this aspect, the solidified sulfur component, after held on the exhaust purifying catalyst, is placed under the high temperature and rich conditions, so as to promote the carrying action of the basic metal onto the exhaust purifying catalyst and enhance the purification of exhaust by the basic metal thus carried on the exhaust purifying catalyst, which can further enhance the exhaust purifying performance.
In a preferred embodiment, an alkali metal element or an alkaline earth metal element is carried on the exhaust purifying catalyst, the basic metal element in the sulfur solidifier is an alkali metal element or an alkaline earth metal element, and the alkali metal element or the alkaline earth metal element in the sulfur solidifier is one having stronger basicity than the alkali metal element or the alkaline earth metal element carried on the exhaust purifying catalyst. In this embodiment, even if gaseous sulfur oxides remaining without being solidified newly flow into the exhaust purifying catalyst, they can be harder to couple with the alkali metal element or the alkaline earth metal element carried on the exhaust purifying catalyst from the beginning. As a result, the original NOx occlusion capability of the exhaust purifying catalyst is prevented from being degraded, and the initial performance of the exhaust purifying catalyst is maintained, so that the exhaust purifying performance can be maintained high.
In another preferred embodiment, an alkali metal element or an alkaline earth metal element is carried on the exhaust purifying catalyst, the basic metal element in the sulfur solidifier is an alkali metal element or an alkaline earth metal element, and the alkali metal element or the alkaline earth metal element in the sulfur solidifier is one easier to decompose than the alkali metal element or the alkaline earth metal element carried on the exhaust purifying catalyst. In this embodiment, during decomposition of NOx and SOx occluded in the exhaust purifying catalyst, the alkali metal element or the alkaline earth metal element becoming newly carried from the sulfur solidifier is first decomposed, which prevents a scattering phenomenon of the alkali metal element or the alkaline earth metal element carried on the exhaust purifying catalyst from the beginning. As a result, it is feasible to maintain the initial performance of the exhaust purifying catalyst and also maintain the exhaust purifying performance high.
A further exhaust cleaning method of internal combustion engine according to the present invention is a method comprising the steps of adding a sulfur solidifier for solidifying sulfur component, to exhaust gas after combustion in a cylinder of the internal combustion engine, to solidify the sulfur component (incl. sulfur oxides) being the cause of SOx poisoning and flowing the exhaust gas after solidification of the sulfur component into an NOx-occluding reduction-type exhaust purifying catalyst.
According to the present invention, the sulfur solidifier solidifies the sulfur component (incl. sulfur oxides) being the cause of SOx poisoning before the exhaust gas flows into the NOx-occluding reduction-type exhaust purifying catalyst placed on the exhaust path, which can suppress the SOx poisoning of the NOx-occluding reduction-type exhaust purifying catalyst and further increase the purification efficiency of exhaust gas. A significant point herein is that the aforementioned sulfur solidifier is added to the exhaust gas after the combustion in the cylinder of the internal combustion engine. This eliminates a need for concern about whether the solidified sulfur component can cause knocking or combustion failure. As a result, it becomes feasible to bring about effective solidification of sulfur component (incl. sulfur oxides) that can be the cause of SOx poisoning of the exhaust purifying catalyst, to ensure improvement in the purifying performance, and also to ensure stable operation of the internal combustion engine.
It is preferable herein to place an oxidation catalyst upstream of the exhaust purifying catalyst on the exhaust path and add the sulfur solidifier to the exhaust gas between the oxidation catalyst and the exhaust purifying catalyst. In this configuration, the sulfur component is oxidized from SO2 to SO3 by the oxidation catalyst and thereafter solidified, which can increase the solidification efficiency of sulfur component.